This invention relates to an improved die assembly for an inline pelletizing extruder.
In the commercial production of polymeric materials, particularly normally solid resinous thermoplastic materials such as polyolefins and the like, the polymer is converted into pellets prior to being packaged and shipped to the fabricator.
These pellets may be produced by extruding strands of molten polymer which are cooled and thereafter cut into short sections. Preferably the strands are severed at the die by a rotating cutter and immediately contacted with water to cool same below the melting point before they touch so as to prevent agglomeration. This can be done by a dry die face cutter surrounded by a cylindrical housing whereby water is impinged on the freshly cut pellets as they swirl around the periphery of the housing. Alternatively the die itself can be in direct contact with the water. This, however, causes larger amounts of heat loss in the die itself which increases the pressure drop across the die. In any event, wherever a technique is used whereby a cutter severs the strands flush with the face of the die to form pellets, it is essential that the die not become warped as this will interfere with the cutting operation.
The problem of warpage of the die is of particular severity in pelletizing high molecular weight polymers such as ultra high molecular weight polyethylene and ultra high molecular weight ethylene copolymers because the high viscosity of the polymer melts generate high back pressures in the extruder barrel and breaker plate area, particularly during startups.